This invention relates generally to continuous casting of metal slabs such as steel and, more particularly, to improved mold plates for such continuous casting.
In the continuous casting of metal slabs such as slabs of steel, a stream of molten metal is typically poured downwardly into a tube-like mold. As the molten metal stream flows through the mold, the mold sufficiently cools the metal so as to form a skin on the outer surfaces of the flowing molten metal. The molten metal enclosed within the skin continues to advance, usually downwardly in a gravity-assisted direction, through cooling zones and once the moving stream of metal is solidified into a continuous slab it may be cut into slab lengths. The initial formation and cooling of the molten metal stream takes place in a mold which typically shapes the metal into a generally rectangular cross-sectional configuration.
Examples of typical size slabs which may be continuously cast in such molds range from between about 8 to 104 inches in width and about 4 to 12 inches in thickness with the slab being cut off the continuously forming slab stream to the desired lengths. Once the metal is solidified, the wider slabs may also be cut longitudinally to form narrower slabs.
In the mold, the rate and thickness of the skin formation varies depending upon the speed of movement of the metal stream through the mold, the cooling temperature of a coolant flowed through the mold walls, the metal composition, etc. By way of approximation, a skin thickness of between about 3/16 of an inch to 1/2 inch may be formed with the skin surrounding a molten metal interior which interior solidifies as the metal advances through subsequent cooling zones exterior of the mold. The molten metal within the skin of the moving stream exerts a considerable ferrostatic pressure that tends to cause the sides of the moving stream to bulge outwardly and often tends to cause the corners of the skin to burst. This is referred to as break-out.
Once the partially solidified moving stream exits from the mold, the metal is conveyed through a cooling zone by rollers of the like and efforts have been made to reduce and/or eliminate the bulging out of the metal stream by the use of rollers engaging the metal just below the mold. These rollers do tend to resist bulging of the metal but the rollers increase the drag or frictional resistance applied to the metal stream, thereby requiring more powerful motors to move the continuous cast metal slab. In addition, because of the bulging or side expansion of the skin by the ferrostatic pressure, it is difficult to hold the dimensions of the metal slab within the desired tolerances without the use of edge rollers or guides.
When the molten metal is in the mold and is in contact with the mold walls, as the skin or outer surfaces of the molten metal starts to cool, the molten metal contracts away from the mold walls. After the molten metal has contracted out of contact with the mold wall, the heat from the interior of the molten metal causes the metal to expand until the metal is again in contact with the mold wall. Thereafter, the metal in contact with the mold wall cools and again shrinks or contracts out of contact with the mold wall. This repetitive contraction and expansion occurs while the molten metal flows downwardly through the mold and thus creates a wavy surface irregularity on the metal slab as well as causing uneven wear on the mold walls.
The prior art patent to Johnson, U.S. Pat. No. 3,910,342, issued Oct. 7, 1975, discloses one technique for minimizing break-out and for providing a slab with flat surfaces. The Johnson patent describes a mold where the interior walls of the mold are convex and where the radius of curvature of the convex mold surface is progressively greater at successive lower levels of the mold passage. The purpose of this configuration is to prolong contact between the flowing metal and the mold walls. The Johnson patent suggests that the radius of curvature at the bottom of the mold may be infinity so that the surface of the metal casting as it leaves the mold will be flat.
U.S. Pat. No. 3,837,392, issued to Rossi on Sept. 24, 1974, is directed to another attempt to solve the problem of non-flat slab surfaces by providing a convex roller to engage the metal strand as the metal strand exits from the mold, the convex roller working in conjunction with cooling sprays to achieve the desired flat surface and cross-sectional configuration of the finished slab. The Rossi patent also discloses the idea of convex configuration of the mold walls in lieu of a convex roller.
U.S. Pat. No. 4,023,612 to Jackson, issued May 17, 1977, describes a technique where flat, inwardly tapered mold wall surfaces have been used and also describes a mold system where flat inwardly tapered mold end walls have a slightly convex intermediate section on each mold end wall. The convex section is uniform along the height of the mold. In a modified version of the apparatus described in the Jackson patent, the side walls are initially flat and thereafter progress from the upper end of the mold to the lower end of the mold with convex sections of progressively increasing height and length.
I have discovered that none of the systems described in these prior art patents provide the desirable solution to the aforementioned problems.